Because text in SVG may have fill, stroke and fill- and stroke-opacity,
it might be a good idea to extent the concept of text-shadow related to
these properties as well for SVG - one may expect, that the shadow of a blue
stroked and red filled text with fill- and stroke-opacity of 0.5 might look
different than the shadow of a black filled text.
Obviously using filters one can care about such advanced expectations in
more detail anyway, but such a simple property might help authors, not
very familiar with filter effects.
Currently for text-shadow the color property seems to be of some relevance,
but I think, in SVG this is not often used - but because this is only a
default value, this is maybe nothing to worry about.
Olaf